Exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that positions a reticle, an optical assembly having an optical axis, a wafer stage assembly that positions a semiconductor wafer, a measurement system, and a control system. The measurement system constantly monitors the position of the reticle and the wafer, and the control system controls each stage assembly to constantly adjust the position of the reticle and the wafer. The features of the images transferred from the reticle onto the wafer are extremely small. Accordingly, the precise positioning of the wafer and the reticle is critical to the manufacturing of high quality wafers.
In certain designs, the measurement system includes an autofocus system that monitors the position of the wafer along the optical axis. Subsequently, with information regarding the position along the optical axis, the wafer stage assembly can be controlled to properly position the wafer along the optical axis.
One type of autofocus system includes a slit light source that illuminates a set of slits and an imaging system that projects the set of slits onto the wafer at a glancing angle of incidence. The light reflected from the wafer is then directed to a slit detector assembly that determines the position of the wafer along the optical axis. Unfortunately, instabilities in the autofocus system and other conditions can adversely influence the accuracy of the autofocus system and ultimately the accuracy in which the wafer is positioned along the optical axis. For example, patterns on the wafer influence the reflectivity of the wafer and can adversely influence the accuracy of the autofocus system. Further, environmental effects such as the refractive index changes of air due to temperature, atmospheric pressure, and/or humidity changes or gradients can adversely influence the accuracy of the autofocus system.